1. Field of the Invention
The present invention relates to a digital TV, and more particularly, to a video converting device and method for a digital TV.
2. Discussion of Related Art
Generally, a digital TV provides an image of a higher quality to a viewer when compared with an analog TV. Therefore, as the interest in digital TVs rise, the technology to compress and transmit video data to a digital TV receiver has been rapidly developing.
FIG. 1 is a block diagram of a typical digital TV. Assuming that the video signal is compressed according to a Moving Pictures Expert Group-2 (MPEG-2) algorithm and transmitted by a Vestigial Side Band (VSB) method, the digital TV includes a tuner 100 which selects one of a plurality of channels received through an antenna and outputs the selected channel to a VSB demodulator 101. The VSB demodulator 101 performs a VSB demodulation of the channel frequency tuned by the tuner 100 and outputs the demodulated channel to a transport inverse multiplexer 102.
In a VSB transmission, two side bands are formed above and below the carrier upon an amplitude modulation of a signal and when one side band is greatly attenuated, only the remaining side band is modulated. For example, a ground wave uses an 8 VSB modulation.
Because the VSB demodulated signal output to the transport inverse demultiplexer 102 is composed of both video and audio signals which are multiplexed in a transport packet type, the transport inverse demultiplexer 102 separates the video and audio signals. The video signal is then output to a video decoder 103 and the audio signal is output to an audio decoder 105.
The video decoder 103 restores the MPEG-2 compressed video signal to its original signal. Namely, the video decoder 103 implements a variable length decoding (VLD) to a pure data, i.e. data of the input video signal excluding overhead such as various header information and start code. Thus, the video coder 103 restores the original pixel values for the screen through an inverse quantization, an inverse discrete cosine transformation, and a movement compensation using movement vectors. The restored pixel values are output to a display processor 104 which converts the decoded video signal to conform with the output format of a display device and displays the video signal through a monitor.
The audio decoder 105 restores the compressed audio signal to its original signal and converts the restored signal into an analog signal, thereby playing the analog signal through a speaker.
However, the digital TV in the related art requires relatively a certain amount of time from the reception to the display of an input signal when there is a change of channel. For example, the digital TV requires a time for converting a signal from the tuner 100 into digital data through the VSB demodulator 101, a time to search for a Packet Identification (PID) corresponding to the audio and video signals in a Program Association Table/Program Map Table (PAT/PMT) or a Program and System Information Protocol (PSIP), a time for the video decoder 103 to begin decoding in conformity with a Decoding Time Stamp (DTS) after storing the bit stream, and a time to decode and display an I (Intra) picture.
Although, a time period of about 2 seconds elapses for the above process, the digital TV still needs a longer time period than an analog TV. Therefore, reducing a time required for changing a channel is important to provide a more convenient viewing to a user.
In addition, a digital TV is capable of transmitting a broadcasting signal in a multi-format. For example, the Advanced Television Systems Committee (ATSC), which sets the standards for the digital TV in the United States, prescribes 18 formats. Thus, a transmission format may change depending upon a program, even for the same channel. Namely, one program may be transmitted in an interlaced format of 1920*1080I while the next program may be transmitted in a 704*480I format.
Furthermore, the format of the input video may differ from the format necessary to display the video. In such case, the video format must be converted to conform to the display format. Thus, if the format of an input video is 720*480I and the display format of the video is 1920*1080I, the 720*480I format must be converted into the 1920*1080I format. If the format conversion is not well processed, a portion of the picture may be momentarily broken and displayed. As a result, accurately converting the format within a short period of time is also important to provide a convenient viewing to a user.
Moreover, the video signal processing Applicable Specific Integrated Circuit (ASIC) for the digital TV uses a memory upon all conversion. Such memory includes three sheets of frame memories which are needed to decode the MPEG-2 video bit stream. The memory is also used to provide an additional display option such as a Picture In Picture (PIP) as shown in FIG. 2.
For example, a memory map for decoding and displaying a high definition (HD) video is shown in FIG. 3A, and FIG. 3B shows a memory map for displaying the HD video on a main screen while displaying externally input video signals on a subscreen such as the PIP. Here, the externally input video signals are all other signals other than a digital signals including the National Television Standards Committee (NTSC) signal or a Video Graphics Array (VGA) signal. Accordingly, the memory map changes depending upon the mode selected by a user. Since the memory maps are changed upon a mode conversion, efficiently processing a mode conversion becomes important.
As discussed above, a conventional digital TV needs a longer tuning time than an analog TV. Also, the picture may be broken upon an input conversion or display format conversion. Finally, the time for a mode conversion is prolonged.
Furthermore, a digital TV should decode and display a digital bit stream, as well as process an external input signal including the NTSC broadcasting signal, the VGA signal and other video signals of a field/frame type. Moreover, when a video signal is not contained in the input signal, the digital TV should execute an appropriate signal processing.
For example, an analog TV displays a video signal, if the video signal is contained in the input signal and displays an arbitrary background color such as the color blue, if the video signal is not contained in the input signal. In other words, if there is no channel signal, i.e. the video signal is not received, or if there is no input signal from the Video Cassette Recorder (VCR), the screen is not directly processed to display a noise, but processed to display a background color, for a more beautiful screen display.
However, a digital TV in the related art simply processes and displays, even a noise, if a video signal is not contained in the input signal. As a result, the digital TV in the related art cannot provide a pleasant viewing experience to a user.